Within the past four years, the genetic cause of the Hereditary Nonpolyposis Colorectal Cancer Syndrome (HNPCC), also known as Lynch syndrome II, has been ascertained for the majority of kindred's affected with the disease (14). The molecular basis of HNPCC involves genetic instability resulting from defective mismatch repair (MMR). Several genes have been identified in humans that encode for proteins and appear to participate in the MMR process, including the mutS homologs GTBP, hMSH2, and hMSH3 and the mutL homologs hMLH1, hMLH3, hPMS1, and hPMS2 (4, 9, 11, 17, 19, 22, 24, 38). Germline mutations in four of these genes (hMSH2, hMLH1, hPMS1, and hPMS2) have been identified in HNPCC kindred's (13). Though the mutator defect that arises from the MMR deficiency can affect any DNA sequence, microsatellite sequences are particularly sensitive to MMR abnormalities (14, 25, 27, 29). Microsatellite instability (MI) is therefore a useful indicator of defective MMR. In addition to its occurrence in virtually all tumors arising in HNPCC patients, MI is found in a small fraction of sporadic tumors with distinctive molecular and phenotypic properties (13).
HNPCC is inherited in an autosomal dominant fashion, so that the normal cells of affected family members contain one mutant allele of the relevant MMR gene (inherited from an affected parent) and one wild-type allele (inherited from the unaffected parent). During the early stages of tumor development, however, the wild-type allele is inactivated through a somatic mutation, leaving the cell with no functional MMR gene and resulting in a profound defect in MMR activity. Because a somatic mutation in addition to a germ-line mutation is required to generate defective MMR in the tumor cells, this mechanism is generally referred to as one involving two hits, analogous to the biallelic inactivation of tumor suppressor genes that initiate other hereditary cancers. In line with this two-hit mechanism, the non-neoplastic cells of HNPCC patients generally retain near normal levels of MMR activity due to the presence of the wild-type allele (11, 13, 24). In addition, similar findings are observed in other diploid organisms (2, 5, 8).
The ability to alter signal transduction pathways by manipulation of a gene product's function, either by over-expression of the wild type protein or a fragment thereof, or by introduction of mutations into specific protein domains of the protein, the so-called dominant-negative inhibitory mutant, were described over a decade ago in the yeast system Saccharomyces cerevisiae by Herskowitz (Nature 329:219-222, 1987). It has been demonstrated that over-expression of wild type gene products can result in a similar, dominant-negative inhibitory phenotype due most likely to the “saturating-out” of a factor, such as a protein, that is present at low levels and necessary for activity; removal of the protein by binding to a high level of its cognate partner results in the same net effect, leading to inactivation of the protein and the associated signal transduction pathway.
Recently, work done by Nicolaides et. al. (32) has demonstrated the utility of introducing dominant negative inhibitory mismatch repair mutants into mammalian cells to confer global DNA hypermutability. There is a need in the art for additional techniques for generating mutations in bacteria
which can be used to make strains for production, biocatalysis, bioremediation, and drug discovery.